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When we talk about code maintainability, we're looking at how easily a codebase can be understood, modified, and extended by developers who may not have been involved in its original creation. Global variables, which are accessible throughout the entire application, can seriously hamper this process. How so? Well, when a function's output depends on some external state that can be changed from anywhere else in the code, it becomes like a puzzle with moving pieces that are hidden from view. For instance, imagine a variable 'userCount' that's global; if it gets altered by various parts of the program, tracking down where and why changes happen can be a nightmare. This makes bug fixing and adding new features a tedious and error-prone process.

For better maintainability, it's essential to minimize global variables and instead pass necessary data to functions as parameters or use encapsulated states within classes or modules. This way, each section of the code is predictable and can be dealt with in isolation, making it easier to grasp and improve upon.

Namespace pollution occurs when too many identifiers, such as variables and functions, exist in a global scope, and it's like a library with books scattered all over the place. With global variables, there's always a danger of accidental name collisions—where two unrelated parts of a program use the same name for different things. This can cause bugs that are difficult to track down. Imagine you've declared a global variable named 'temp' to store temperature data. Elsewhere in the code, another developer uses 'temp' as a temporary variable for a calculation. The program becomes unpredictable and difficult to reason about.

To avoid such issues, use local variables within functions or other scope-limited constructs whenever possible. Namespaces, classes, and modules are great tools to segregate and organize code logically. By doing so, you not only reduce the chance of clashes but also make your code self-documenting; it's clearer at a glance what each part of the codebase is supposed to do.

Modularity is the breakdown of a program into smaller, manageable, and interchangeable parts or modules. It aligns with reusability, which is the practice of writing code that can be used again in different parts of a program or even in different programs altogether. Global variables are usually at odds with both these concepts. They introduce a level of entanglement that makes it tougher to lift a function or a module out of one project and drop it into another. Why? Because functions that rely on global variables are inherently less portable; they assume those globals exist and hold certain values.

To promote modularity and reusability, it's key to design functions and modules with clear input and output interfaces, with no side effects caused by or causing changes to global state. Packaging functionality in such a way encourages reuse. You can then take a logging module, for example, and plug it into a new project with minimal effort, confident it will perform its job without unforeseen complications.